The system profiler should poll the hardware and software configuration of the computer to generate the report of the current state of the system; however, a few Snow Leopard users have found that when some firewall settings (specifically stealth mode and logging) are changed they do not appear to be reflected in the System Profiler. Instead, regardless of the settings the System Profiler claims the settings are not enabled, which has confused a number of people.
It seems there is a bug in the system profiler, which has it incorrectly reading the firewall settings. There are two instances of system profiler in OS X, the first is in the system profiler application itself, and the second is the equivalent command-line utility, and both of these will incorrectly report the settings.
Despite what the system profiler says, the firewall settings do work. To test this, make note of your computer's IP address (in the "Network" preferences) and then open Snow Leopard's "Security" system preferences and go to the firewall section. Then from another computer on the same network ping the Snow Leopard system by entering the following in the terminal for the second computer:
You should see a list of responses from the system as long as stealth mode is not enabled, but upon enabling it the responses should stop, indicating the firewall is working even though this is not reflected in the system profiler.